The use of aqueous based fluids to fracture subterranean formations to increase the flow of fluids therefrom (e.g., oil, gas, water, etc.) is a well-known art. In many instances the aqueous based fluid is treated to increase its viscosity and solids carrying ability to increase the efficiency of the process. In one such process (Kern, U.S. Pat. No. 3,058,909) a complex of a polyorganic compound having at least one reactive unit consisting of two adjacent hydroxyl groups arranged in a "cis" configuration, e.g., a galactomannan gum, and a boron compound capable of supplying borate ions in the aqueous solution are added to the aqueous fluid. In the Kern process, however, holding tanks and critical time periods are involved since it was thought at that time that the gum must be permitted to hydrate for an appreciable length of time in the solution at an acidic pH prior to crosslinking the gum with borate at a basic pH value. The reasoning exemplified by Kern was based on the following known facts. First, a galactomannan gum will not hydrate when it is dispersed into an alkaline aqueous solution which contains borate ions. This characteristic is sometimes employed to aid in dispersing the gum since its tendency to agglomerate is reduced. Also, it is known that if the pH of an alkaline solution containing a gum and borate ions is adjusted to an acidic value, the gum will hydrate but it will not crosslink to form a gel. However, after the gum is permitted to hydrate at an acidic pH value it will become crosslinked if the pH of the solution is then shifted back to a basic value. Therefore, those skilled in the art, e.g., Kern, have taught to first disperse the gum into an acidic aqueous fluid containing borate ions; allow it to hydrate; add a base, e.g., NaOH, to adjust the pH to a basic value after the gum has hydrated; and only then can the boron compound crosslink the gum. This process is successful for small scale uses. However, in oil or gas fields where such aqueous fluids are to be employed as fracturing fluids in large volumes, holding tanks and the time delay involved may rule out such a process because of economic and other considerations, e.g., size of holding tanks required and the like.
However, borate crosslinked galactomannan aqueous fluids are very useful in fracturing techniques. Therefore, it would be desirable if a method were available for employing the borate-gum system in a continuous, i.e., "on the fly" method so that while an aqueous fluid is being introduced into and through the borehole which penetrates the subterranean formation, the gum could be hydrated and crosslinked. The present invention concerns such an on the fly process.
Other relevant art includes U.S. Pat. Nos. 3,096,284; 2,868,664; 3,615,794; 3,740,360; 3,763,934 and 3,766,984, the teachings of which are specifically incorporated herein by reference.